Piston & connecting rods

81slantnosewp

Well-known member
Connecting Rods:

The 240 rod length is specified at 6.7947” and the 300 rod is 6.2097”. They are generally rounded up to 6.8" and 6.21"
The 1965-1968 240 and 300 rods have a .912” wrist pin diameter. 1969 and later rods have a .975” pin and an oil spit hole in the big end of the rod.
All rods are .992” wide at the big and small end.
Rod journals are 2.123”

The early 240/300 rods without the spit holes are preferred for high rpm performance builds and may be used with Ford 302 V8 pistons for the 240 and Ford 351 V8 pistons for the 300 because of the .912" wrist pins.

The early 240 rods (forging number 65AE) may be used with the 300 crankshaft with a custom piston for significant piston and pin weight reduction.

The small 2.100” journal BBC rods have a .992” wide big end can be used with custom pistons.
These rods use the SBC 2.100" bearings.
The 300 crank rod journals are turned down form 2.123" to 2.100" to accommodates the BBC small journal rods.
The 6.385” long small journal BBC rod is available as an “off the shelf” item.
Molnar and Compstar make the lowest cost versions of theses rods.
Molnar CH6385NTB8-A
Compstar CSB6385DS3B4AH
https://www.summitracing.com/parts/cpi-b6385ds3b4ah

It has become apparent that the early 240 rods are becoming extinct and the need for a reliable performance connecting rod continues to increase.

There was a previous discussion in DEC of 2018 where a custom 6.430" long rod would accommodate a lot of the Chevy 350 V8 pistons.
https://fordsix.com/viewtopic.php?p=612046#p612046

As you may know the Chevy LS rod Does Not have an offset beam as does the earlier generation SBC engines.
Eagle carries LS rods in 6.460" and 6.560" lengths for a 2.100" rod journal. CRS6460O3DL19 and CRS6560O3D2000
These two lengths give access to many of Chevy 350 pistons.

What is up for discussion (among other things)is the fact that the LS rod is .940" wide at the big end and the 300 rod is as wide as .992" at the big end.
That difference adds .052" to the rod's side play clearance.
Oil pressure is not affected by connecting rod side clearance making the Eagle LS rod a possibility.

For those that want to build a very high rpm 240 including turbocharging where the rpm is only limited by the valve train.

Molnar makes a 7.130" long billet rod for the Chevy 292 six that will fit the 240 crank.
The 240 crank journals are turned down to 2.100" and the journals are widened .035" so a generous radius can be formed for extra crank strength.
https://www.12bolt.com/store/p32/292_Bi ... olnar.html

s948739236638414765_p32_i2_w640.jpeg


Use a custom forged piston that has a 1.28" CD with a total piston and pin weight under 600 grams and have a screaming six.Pistons:


The 1965 to 1968 240/300 engines have a .912" wrist pins.
The 240 from these years uses the Ford 289/302 V8 pistons and the 300 can use the Ford 351W pistons.
You will find forged and cast pistons among the variety of V8 pistons.

The 1968 and later engines have a .975" wrist pin.
The Ford 352 V8 pistons may be used if some material is removed off the piston top.
The Ford 390 V8 pistons may also be used but requires an .050" overbore.

There are cast pistons specifically made for the 300 six.
The Hypereutectic cast pistons are preferred over the other cast pistons.
There aren't any catalog forged pistons with the .975" wrist pin.

The following is a list of cast pistons with the .975" pin pistons

240 cast piston:
KB/Silv-O-lite 1147

300 cast pistons:

Speed Pro H519P
Speed Pro H554CP
Speed Pro H674P (metric rings)
Silv-O-Lite 1170
KB/Silv-O-lite 1186
KB/Silv-O-lite S3171H (metric rings)
KB/Silv-O-lite 3117H
KB/Silv-O-lite 3118H
KB/Silv-O-lite 3158H

if you happen to have a 240 and connecting rods with a .912 pin
forged probe pistons part number 8100-2-10684 w/1.6 comp. height

AutoTec pistons is the main source for custom forged pistons for the 240/300 six.
They use 4032 forged alloy aluminum.
They are the least expensive forged piston manufacturer.

If you are going to go heavy on Nitrous, Supercharging or turbocharging, most of the piston suppliers will make you custom 2618 alloy pistons.

click her for critical engine dimensions

my personal combination currently is
240 connecting rods
probe forged pistons 8100-2-11938
zero decked block
[image]http://i.imgur.com/rcoY6v6.jpg[/image]not perfectly accurate put close
8100-2-10662-2.jpg

probe pistons are currently being sold through vigilanteparts and are being subcontracted by i believe dss, ross or autotec depending on volume email to get a quote for six rather than 8 pistons organize a group buy to save money
 

Firepower354

Famous Member
I've not seen a reason why re-bolted 292 rods and 1.240ch ICON or similar wouldn't be good. They have 5cc and 26cc .927 pin, metric rings.
Or 1.250 from 6" rod 350 putting it a tad proud of deck, but HG thickness is .050 on the HD, I think.
I'm sure Cometic will whip up whatever is desired. Surprised that nobody has had them done. I've had a couple oddballs scanned in.
 

THE FRENCHTOWN FLYER

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Firepower354":1vq6njai said:
...I'm sure Cometic will whip up whatever is desired. Surprised that nobody has had them done...

I've had them make me a couple.

$200
They have the prints; now all they need is your money.
 

curts56

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Stock on the left, Compstar in the middle, and Super Rod aluminum on the right (no longer available).

20200324010725-c546a691-la.jpg
 

pmuller9

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benchracer":1l3st269 said:
Could we destroke our 300 to a better piston and rod combo?
You can off set grind the iron crank using a smaller rod journal.
Any significant journal changes require welding and regrinding and you really need a steel crank to do that.

It is far easier and cheaper to use a custom piston to accommodate a different connecting rod.
 

sixtseventwo4d

Well-known member
Focusing on just the 240 vs. 300 rod use, and aside from pin weight; it would seem an advantage to use the 300 rod since it would actually change the location in the cylinder where the piston accelerates thus allowing the cam more advantage, also allowing for a longer ramp profile. The cylinder swept volume would remain but since the piston would be moving faster as it nears the opposite ends of travel this seems to create a faster rapping engine. This is just something I've been focusing on today since I'm contemplating my own build. I'm interested in others input. I hope I'm posting in the correct area for this discussion.


Edit: Although i do think the 300 rod would contribute to increased piston/cylinder side loading on the compression/combustion stroke.
 

sqrbckguy38

New member
So I found a set of Fe pistons in the garage that were actually STD bore, and made my own tool to remove the wrist pin. So I've been pairing up pistons and rods to get weighted, and the block bored for the Fe pistons. The Fe rod I noticed had a bushing in the small end, where as the i6 does not. What would be the harm in perhaps installing one? These pistons have a clip for the wrist pin, I was debating making the rods a floating design. Any thoughts on drawbacks with this?
 

pmuller9

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There is nothing to be gained by going through the expense of making a floating piston for what you are doing.
What is the piston CD?
 

sqrbckguy38

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Not sure. But there's not really any expense to this adventure. All I asked was about any issues with doing so. Being that it's like 20 a rod to press pistons on or off, it would be great for saving some money. With the wrist pin between the two pistons, they sit perfectly level at the top. But the i6 is slightly shorter on the bottom of the skirt. I'll send the block to get match bored later next week. Stock rods, Fe pistons.
 

pmuller9

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sqrbckguy38":31k1pzb9 said:
Not sure. But there's not really any expense to this adventure. All I asked was about any issues with doing so. Being that it's like 20 a rod to press pistons on or off, it would be great for saving some money.
I didn't know it cost you that much to press piston pins.
In that case bush the rods and save some money.
 

sixtseventwo4d

Well-known member
sqrbckguy38":2vh8ufsr said:
So I found a set of Fe pistons in the garage that were actually STD bore, and made my own tool to remove the wrist pin. So I've been pairing up pistons and rods to get weighted, and the block bored for the Fe pistons. The Fe rod I noticed had a bushing in the small end, where as the i6 does not. What would be the harm in perhaps installing one? These pistons have a clip for the wrist pin, I was debating making the rods a floating design. Any thoughts on drawbacks with this?

The idea of bushing the rod is definitely worth giving thought. I'm going to suggest that there would be some weakening of the small end by doing so since you are in fact removing material . As far as cost, it sounds expensive between boring the rod end, pressing the bushing and then fitting the pin to the bushing bore; all which would need to be very accurately done or the piston and rings will be wearing prematurely. It would be nice to fit our own pistons without running to have them pressed each time. :beer:
 

Lunatic Fringe

Well-known member
[image] [/image]
Rod on the left is a 400 we use in the V8's with a bushing to go from .975 to .912 to use 351C popup pistons. Rod on the right is a GRP Billet BBC .400 long w/SBC journal size that I'll be using in our 300. Small end is bushed from .990 BBC size down to match the pistons that we'll use.
 

pmuller9

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sixtseventwo4d:7wb5a1ey said:
Focusing on just the 240 vs. 300 rod use, and aside from pin weight; it would seem an advantage to use the 300 rod since it would actually change the location in the cylinder where the piston accelerates thus allowing the cam more advantage, also allowing for a longer ramp profile. The cylinder swept volume would remain but since the piston would be moving faster as it nears the opposite ends of travel this seems to create a faster rapping engine.
Edit: Although i do think the 300 rod would contribute to increased piston/cylinder side loading on the compression/combustion stroke.
Somehow I missed this post.

The 240 rod used on a 300 crank allows a 30% reduction in piston and pin weight.
Rod and main bearing life is increased.
The piston no longer protrudes past the bottom of the cylinder wall.

300%20piston%201.JPG
 
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THE FRENCHTOWN FLYER

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Shorter rods were once the rage in drag racing for an application where the cubic inch displacement was reduced by de-stroking, to make a class requirement for a specific cubic inch-to-weight.

This was done in an effort to enhance the flow in the overlap phase, where port and valve sizes were very large in comparison to the (reduced) cylinder displacement, leading to lazy flow characteristics during overlap.

The situation in our 300 six is quite the opposite - we have a relatively large cylinder to fill and purge with a relatively small set of ports so there is really no advantage to keeping the shorter rod.

Once again I would like to paraphrase one of my engine building heroes Buddy Morrison of Reher Morrison fame, " If I were to list the most important factors in the construction of a race engine rod length would rank about number 50."
 

sixtseventwo4d

Well-known member
Thank you pmuller9. I needed that clarification, some posts on here have got that number muddled with 65AE. When I tried giving that number to a core supplier I like to use he said it wasn't good for him.
 
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